The present invention relates to an improved apparatus for exposing a photosensitive coating on the faceplate of a cathode-ray tube to light to form a phosphor screen on the faceplate, the improvement relating to means for causing the light to follow simulated electron-beam trajectories more accurately.
Cathode-ray tubes (CRTs) are widely used at present to display images in television sets, computer monitors, and the like. The displayed image is created when the phosphor screen on the faceplate of the CRT is scanned by an electron beam emitted from an electron gun. In a color CRT, electron beams from different electron guns scan the screen simultaneously, landing on phosphors of different colors. To ensure that each beam lands on phosphors of the proper color, the phosphor screen is formed by exposing photosensitive phosphor materials to light through the slotted color separation mask through which the electron beams will also pass, using a main lens with a specially shaped surface that refracts the light into paths simulating the electron-beam trajectories.
If the electron beams do not land in the right positions, problems such as color smear may occur. The causes of beam-landing error include environmental factors such as the ambient temperature, and manufacturing-process factors such as misalignment of the electron guns. The manufacturing-process factors are difficult to eliminate completely, but fortunately, they can be compensated for.
One conceivable method of compensation would be to modify the surface figure of the main lens, or grind a new lens, whenever the manufacturing process was found to lead to beam-landing error. This approach is impractical.
A more practical method is to insert a corrective lens, referred to as a sub-lens, in the light path, either in front of or behind the main lens. A set of sub-lenses with different surface configurations can be prepared in advance, and inserted singly or in combination to compensate for various types of beam-landing error. A known set of sub-lenses is described in Japanese Unexamined Patent Application No. 10-83161.
The present inventors have found, however, that the sub-lenses in this known set fail to compensate for certain observed beam-landing patterns. Furthermore, the insertion of extra sub-lenses in the light path interferes with the simulation of electron-beam trajectories. Details will be given later. The overall result is that the prior art of beam-landing error compensation in the CRT manufacturing process lacks the desired degree of accuracy.
An object of the present invention is to compensate accurately for factors in the manufacture of a CRT that lead to beam-landing error on the phosphor screen of the CRT.
The invention provides an apparatus for exposing the faceplate of a cathode-ray tube to light, thereby patterning a phosphor screen on the faceplate. The apparatus includes a light source, and a lens group disposed between the light source and the faceplate. The lens group comprises a fixed lens having a curved surface that refracts light in simulated electron-beam paths, and at least one replaceable optical element. The replaceable optical element is normally a flat transparent plate, but the flat transparent plate can be replaced by a sub-lens having a curved surface, to compensate for inaccuracy of the simulated electron-beam paths. The flat transparent plate and the sub-lens have substantially the same thickness.
Due to their substantially equal thicknesses, the effect of the replacement of the flat transparent plate by the sub-lens is limited essentially to the effect of the curved surface of the sub-lens; no unintended shifting of light paths is produced by the flat body of the sub-lens. The sub-lens can accordingly compensate for a beam-landing error pattern without adversely affecting the overall simulation of electron-beam trajectories by the main lens.
The invention also provides two specific types of sub-lens. One has a cosine surface profile with bilateral symmetry. The other has a surface described by a mathematical function that includes a product of a linear factor in one coordinate and a sine factor in another coordinate. These sub-lenses compensate for beam-landing error patterns that are dealt with inadequately, or not at all, by the prior art.